1. Field of the Invention
The present invention relates to an uplink signal receiving method and apparatus using successive interference cancellation (SIC) in a wireless transmission system based on Orthogonal Frequency Division Multiple Access (OFDMA).
2. Description of the Related Art
In general, Orthogonal Frequency Division Multiplexing (OFDM) is a technology for used carrying information to be transmitted on a plurality of mutually orthogonal sub-carriers.
OFDM is similar to Frequency Division Multiplexing (FDM) in that it makes use of many sub-carriers. Further, OFDM allows for spectrum overlapping between the sub-carriers due to their mutual orthogonality, and has a high bandwidth efficiency compared to FDM.
Further, an OFDM transmission system uses an OFDM symbol whose length is considerably longer than the impulse response of a channel, so that it is resistant to multi-path fading. In addition, the OFDM transmission system has a long symbol compared to a single carrier system, so that it is advantageous for high-speed transmission.
The conventional transmission system based on OFDM generally includes an OFDM transmitter and an OFDM receiver.
The OFDM transmitter converts raw data to be transmitted into an OFDM symbol, and caries the OFDM symbol on a radio frequency carrier. The OFDM receiver receives the OFDM symbol transmitted by the OFDM transmitter of the terminal, and restores the raw data transmitted at a transmission stage.
In the commercialized transmission system based on OFDM, it is more difficult to implement the receiver than the transmitter, and the performance of the receiver exerts a greater influence on transmission performance of the entire system than the performance of the transmitter. This is because the transmitter leaves no room for distortion of a signal, and thus generates an OFDM symbol having a high signal-to-noise (S/N) ratio, while the receiver needs to use a complicated signal processing algorithm for restoring a signal distorted by a wireless channel having multi-path properties and imperfect analog components. Furthermore, the signal processing algorithm needs to be varied depending on each system.
In general, the performance of a receiver improves as the complexity of its signal processing scheme increases. However, receivers with complex signal processing schemes are difficult to implement, and there is an increase in the size of their semiconductor components and consumption of power.
On generating the OFDM signal, because desired data can be carried on each sub-carrier, the OFDM system can be used as a multiple access system. The multiple access system is referred to as an Orthogonal Frequency Division Multiple Access (OFDMA) system.
In a wireless network to which the conventional OFDMA is applied, a downlink signal is generated only by the transmitter of a base station, and each terminal receiving the generated downlink signal decodes the received signal, and extracts only its own information.
An uplink signal received by the base station is the sum of signals generated by the terminals, each assigned a different sub-carrier and a different symbol interval. For this reason, reception performance of the base station can be lowered.
More specifically, there is a difference in a reference clock frequency used by different terminals to generate the OFDM signal, and thereby orthogonality between the sub-carriers constituting the uplink signal is easily disrupted.
A typical problem caused by a reference clock frequency difference between the terminals is mismatch between transmission carrier frequencies of the terminals.
It is assumed that the uplink is assigned to two or more terminals, the sub-carriers assigned to first and second terminals are alternately located, and the length of a symbol generated by each terminal is constant and indicated by T.
When a sub-carrier interval between neighboring sub-carriers is given by Equation (1(, orthogonality is maintained.Δf=1/T   (1)where T is the symbol length of the transmission carrier frequency signal.
It can be found that, although each sub-carrier has complete orthogonality in each terminal, the transmission carrier frequency signals of terminals do not match, and thus Δf is not maintained between the carriers in the uplink signal, the sum of the signals of the terminals.
When an offset between the transmission carrier frequency signals of the terminals making up the uplink signal takes place, orthogonality between sub-carriers making up the uplink signal is distorted, and thus interference between sub-carriers results, which directly deteriorates reception performance.